An in situ experiment finds that reducing the acidity of the seawater surrounding a natural coral reef significantly increases reef calcification, suggesting that ocean acidification may already be slowing coral growth. See Letter p.362
In 1770, the only external threat to Australia's Great Barrier Reef was localized damage caused by European sailing ships such as HMS Endeavour, which struck the reef on 11 June that year. Wind the clock forward almost 250 years and the threats to tropical coral reefs worldwide have escalated to a level that imperils the survival of these complex, diverse and beautiful ecosystems1. On page 362 of this issue, Albright et al.2 report an elegant field experiment in which they measured the response of a coral-reef community in the southern Great Barrier Reef to ocean chemistry conditions that were characteristic of the pre-industrial era. By turning back time in this way, they demonstrate that, all else being equal, net coral-reef calcification would have been around 7% higher than at present, suggesting that ocean acidification may already be diminishing coral-reef growth.
Threats to coral reefs range from regional factors, such as overfishing and land-based pollution, to global-scale issues arising from human interference in Earth's climate system, causing warming and acidification of the oceans. Tropical coral reefs have shown their sensitivity and vulnerability to the relatively modest amount of warming that has already occurred. This warming has, for example, resulted in mass coral-bleaching episodes on many of the world's reefs, most dramatically during the powerful 1997–98 El Niño event, and again during the equally large 2015–16 El Niño. Alongside this warming is the ongoing acidification of the oceans, caused by increased uptake of carbon dioxide from the atmosphere. Acidification is projected to increasingly compromise the ability of calcifying marine organisms to produce the calcium carbonate that forms their skeletons and shells3.
The skeletons of coral animals form the backbone of tropical coral reefs and provide habitat for many thousands of other organisms. Calcification rates must be fast enough to withstand the natural forces of physical and biological erosion. Coral animals achieve this with the aid of the extra food and energy provided by single-celled symbiotic algae called zooxanthellae. These algae provide the beautiful colours of corals, but under stressful environmental conditions, such as unusually warm water temperatures, the algae are lost from the coral tissue, resulting in bleaching.
Corals build their skeletons by precipitating calcium and carbonate ions from seawater, but the absorption of CO2 by seawater shifts the equilibrium away from carbonate-ion availability towards bicarbonate ions. The world's oceans absorb about 25% of the extra CO2 that humans inject into the atmosphere — without this sink the planet would have warmed more than it has so far. But this absorption has decreased the pH, carbonate-ion concentration and saturation state of carbonate minerals of the surface oceans, collectively referred to as ocean acidification4.
Since alarm bells starting ringing about the probable consequences of these changes for tropical coral reefs5, numerous laboratory and field experiments have confirmed that calcification rates decline with ocean acidification. However, these studies have focused primarily on single coral species in experimental environments manipulated to mimic future conditions. The beauty and novelty of Albright and colleagues' study is that they restored the ocean chemistry of a natural reef to that of pre-industrial times, thus factoring out other potentially confounding factors, such as temperature.
The authors took advantage of the unusual hydrodynamic environment at One Tree Reef in the southern Great Barrier Reef (Fig. 1). Here, three lagoons are isolated from the ocean at low tide, but water flows between two of the lagoons (as a result of elevation differences) across a well-developed reef flat. For 60 minutes at low tide each day for 22 days, the researchers pumped a non-reactive dye solution across the reef flat, which for 15 days was enriched with sodium hydroxide to increase the alkalinity, and thus reduce the acidity, of the water.
This application of dual tracers — alkalinity as an active tracer and the dye as a passive tracer — has been previously applied to atmospheric aerosols. The tracing allowed the authors to calculate how much of the added alkalinity was taken up by the reef-flat community (around 17%). The authors also recorded an increase in aragonite saturation state (a measure of the availability of dissolved carbonate and calcium ions) of around 0.6 and an approximately 7% increase in net coral-community calcification, findings that closely match results from previous experimental manipulations of ocean chemistry6.
One implication of these discoveries is that ocean acidification may already be contributing to observed declines in coral growth7. As Albright et al. acknowledge, ocean chemistry is not the only influence on coral calcification. The surface temperatures of the tropical oceans were also significantly cooler in pre-industrial times8. Although current rates of warming are deemed detrimental, some corals may have initially benefited from warmer oceans7. We need to consider not just what ocean acidification and warming separately mean for coral-reef calcification, but also their combined effects and how these factors interact with local stressors, as well as the consequences for coral-reef communities as a whole. Extension of Albright and colleagues' approach to include other factors could help us to get to grips with the relative risks of a rapidly changing future for different coral reefs and locations.
By turning back time in an experimental scenario, Albright et al. have demonstrated the vulnerability of the process of reef calcification to ocean acidification. But we cannot turn back time for the world's tropical coral-reef ecosystems; we have already committed them to a warmer and more acidic future that is likely to fundamentally change them from the diverse and beautiful ecosystems of the pre-industrial era to much simpler ecosystems that are not dominated by corals9. Such a loss will also affect many millions of people who depend on coral reefs for their livelihoods and food security.
The 2015 Paris climate agreement to keep the average global temperature “well below 2 °C above pre-industrial levels” contends that this limit “would significantly reduce the risks and impacts of climate change”. But a 2 °C limit may not be enough for tropical coral reefs, which are at high risk of substantial impacts from ocean warming and acidification, even with the most stringent emission targets10.Footnote 1
Hoegh-Guldberg, O. et al. Science 318, 1737–1742 (2007).
Albright, R. et al. Nature 531, 362–365 (2016).
Doney, S. C., Fabry, V. J., Feely, R. A. & Kleypas, J. A. Annu. Rev. Mar. Sci. 1, 169–192 (2009).
Jiang, L.-Q. et al. Glob. Biogeochem. Cycles 29, 1656–1673 (2015).
Kleypas, J. A. et al. Science 284, 118–120 (1999).
Chan, N. C. & Connolly, S. R. Glob. Change Biol. 19, 282–290 (2013).
Lough, J. M. & Cantin, N. E. Biol. Bull. 226, 187–202 (2014).
Tierney, J. E. et al. Paleoceanography 30, 226–252 (2015).
Fabricius, K. E. et al. Nature Clim. Change 1, 165–169 (2011).
Gattuso, J.-P. et al. Science 349, 45 (2015).
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